1. Technical Field of the Invention
The invention relates to a contact temperature sensor for determining the surface temperature of an object of measurement, a contact plate with a temperature sensitive element being pressed, thermally isolated, against the object of measurement.
2. Description of the Invention Background and Prior Art
A contact temperature sensor for pipelines is known from DE-A-30 38 956. Here, the temperature sensitive element is in a contact part with variously curved outer walls. In dependence upon the form of the object, a corresponding outer wall is selected and, with the sensor, is pressed closely against the measurement object. It is disadvantageous that the contact sensor is not suitable for variable outer surfaces, it does not permit point measurements and requires a long measurement time.
DE-A-40 39 339 describes a contact temperature sensor with which a plane contact plate, with a measuring element, is through under-pressure attached by suction flush to the object surface. The contact plate is part of a displaceable sensor and sealing arrangement, which can also tilt to a slight degree. In this way, a better placement of the contact plate against the object is achieved than is otherwise achievable by pressing against, screwing to, spring loading against or with the use of magnets. The technical outlay involved for the under-pressure arrangement is, however, considerable.
DE-OS 40 39 336 discloses a temperature measurement head for a coordinate measurement apparatus. Formed as a contact temperature sensor, the measurement head serves for rapid workpiece temperature measurement. It consists of a sleeve, fixed to a housing, and a spring-movable deflectable part at the lower end of which a ball joint forms a second bending point. The ball joint comprises a sensor part having a plane contact plate and a temperature sensitive measuring element. The second bending point in the measurement head system is intended to ensure a flat placement of the contact plate against the measurement object surface. Through the additional ball joint bending point, inclined dispositions of the work piece surface with respect to the axis of the measurement head system of up to 10 degrees can be compensated. It is, however, disadvantageous that the additional ball joint bending point is technically complicated and quickly becomes stiff.
The reason for the unreliability and deficient measurement accuracy of hand-held devices is not clear. Probably, however, with hand-held devices the contact plate is not placed sufficiently flush on the object surface. On the one hand, the contact plate must not be too small, since otherwise even the smallest contamination or deposit could disproportionately affect the heat flow to the contact surface. Also, a visual check of whether the contact plate is in fact placed flush or is tilted would not be possible. Further, small contact plates are mechanically less robust, and easily bend, since contact is often accomplished with a relatively high bearing pressure. Hand-held devices with small contact plates are therefore only accurate when new and are not really practicable.
On the other hand, if the contact plate is formed to be relatively large, point measurements can no longer be obtained. Further, the measurements require more time, as a result of the greater thermal capacity of the plate. It is also a disadvantage of large contact plates that sufficiently large plane areas on the object are often not available. There is thus a requirement that the contact plate of the contact temperature sensor have a breadth approximating that of a pencil, i.e. between 0.2 and 1.2 cm diameter.
With such hand-held devices it is to be noted, however, that they do not lead to accurate measured values even when they are provided, as above, with one or more joints serving the flush placement of the contact plate. This is partly the result of the fact that, over the necessary measurement time of ca. 5 to 30 seconds, the hand cannot be held still or begins to tremble, as a result of continual body movements. This leads continuously to small tiltings of the contact plate, and the disturbance of the uniform flow of heat to the contact plate. Conventional joints between the gripping part and the contact plate cannot overcome this problem, since the forces involved in tilting are very small when a small bearing area is involved. If the contact plate is more firmly pressed on, the trembling of the hand also increases. Moreover, the trembling movements of the hand are of many different kinds and of large dynamic range, such that they cannot be countered with conventional joint constructions.